Stents are known from the state of the art in many different forms. They are used inter alia in connection with percutaneous transluminal angioplasty (PTCA=percutaneous transluminal coronary angioplasty) in vascular surgery. Stents however can also serve to dilate other openings in the body or to hold them in a diluted state. Those medical procedures firstly presuppose that the location of the constriction in the respective vessel has been determined. Then for example in PTCA a so-called angioplasty balloon is pushed into the artery which has the constriction, the so-called stenosis, and moved to the location of the stenosis. The balloon is then expanded so that the radially outwardly directed force of the balloon dilates the stenosis and in the optimum situation restores the original passage cross-section of the previously constricted artery.
The stents set forth in the opening part of this specification were developed to maintain the dilated condition of the artery subsequently to the dilation operation. The stents thus serve to prevent the recurrence of a stenosis. The success of so-called stenting however depends inter alia on how uniformly the stent can bear against the inside of the vessel wall. For, the more uniformly the vessel wall can be supported by the stent, the more probable it is that vessel constrictions cannot occur again in the region of the installed stent. Accordingly, a uniform and regular stent structure produces a relatively smooth inside surface in the vessel, to which blood particles can stick only with difficulty. In addition, proliferation of the intima into the interior of the vessel through the stent is also prevented to a greater degree by a uniform and regular stent structure.
Stents having the above-mentioned closed structure are known from the state of the art. U.S. Pat. No. 4,655,771 for example discloses a stent, a so-called wall stent, having a closed structure which is formed from two wires which are woven in a uniform mesh-like configuration and which extend helically on the longitudinal axis of the stent.
The advantage of the closed structure in terms of covering the inside wall of the vessel is however at the cost of the disadvantage that those stents involve relative longitudinal stiffness during insertion of the stent in particular into narrow blood vessels. Those stents therefore do not make it possible in the optimum fashion for the stent to be introduced through possibly very sharply curved vessel portions of the coronary blood vessels, upon being introduced in a direction towards the stenosis to be treated. Closed structures also give rise to problems in regard to using them in the region of curved vessel portions, by virtue of their longitudinal stiffness. A particular disadvantage with a closed structure like the wall stent moreover is that substantial longitudinal contraction occurs upon dilation.
In order to avoid the disadvantages with stents having a closed structure, stents have been developed which are of a so-called modular structure. In those modular-structure stents, individual portions which are provided with a closed structure are connected together by flexible connections. Stents of that kind are known for example from U.S. Pat. No. 5,104,404.
A stent of the general kind set forth is also known by the name Tenax® (Handbook of Coronary Stents, Martin Dunitz, Ltd, London, 1998, pages 121 ff). In that stent the adjacent edge regions of adjacent tubular portions extend in displaced relationship with each other through half a period in the peripheral direction of the stent for reasons of compensation in respect of length. That provides for compensation for the reduction in length of the stent, which results upon expansion from the reduction in length of the cell-shaped elements in the longitudinal direction of the stent.
A disadvantage with the known modular or segmented stents however is that often in the expanded condition they have relatively large gaps in the covered region of the wall of the vessel, while it is precisely segmented stents that also have such gaps of different sizes. Because the inside wall of the vessel is covered by the webs or bars of the stent in a more or less irregular fashion in the expanded condition of the stent, and in particular also due to the relatively large areas that remain free between the bars or webs of the stent, the tissue of the inside wall of the vessel, after implantation of the stent into the vessel, can suffer from prolapse, which can lead to renewed constriction of the vessel, a so-called re-stenosis.